where \(u\stackrel{K}{\rightarrow } x\)  means \(u\rightarrow x\)  while \(u\in K\) ,
represents the slope of \(\varphi \)  at \(x\)  restricted to \(K\) .
It may be regarded as a restricted version of the well-known
notion of (strong) slope of a function \(\varphi :X\longrightarrow \mathbb {R}\cup \lbrace \pm \infty \rbrace \)  defined on a metric space \((X,d)\) ,
denoted here by \(|\nabla \varphi |(x)\) , which was introduced in
[1]} and subsequently employed in various contexts of variational
analysis (see, among the others, [2]}, [3]}, [4]}, [5]}).
